1. Field of the Invention
The present invention relates to a high-voltage transformer mounted on a circuit board of various types of electronic device, and more particularly, to a high-voltage transformer suitable for use in a DC/AC inverter circuit which causes a plurality of cold-cathode discharge lamps (CCFL) for a backlight of various types of liquid crystal display panel used for a notebook personal computer or the like to discharge and turn on.
2. Description of the Prior Art
Conventionally, as a technique for providing insulation between windings of a high-voltage secondary winding of a high-voltage transformer used for an inverter circuit, a technique of forming a plurality of partition flanges on an outer surface of a secondary-side roll to divide a winding area of the roll into a plurality of winding sections to thereby reduce a potential difference between windings in each winding section is known.
A conventional high-voltage transformer to which such a technique is applied is constructed in such a way that a top layer of a secondary winding wound around one winding section is substantially flush with a top layer of a secondary winding of the neighboring winding section on both sides of a partition flange. Furthermore, since a groove is formed in the partition flange to pass the secondary winding whose winding around one winding section has been completed to the next winding section, the top layer of the secondary winding wound around the one winding section is designed to be located extremely close to the top layer of the secondary winding of the neighboring winding section with respect to the location of the groove. In such a structure, increasing the number of windings of the secondary winding in each winding section causes a voltage difference between neighboring winding sections to increase, raising the likelihood of dielectric breakdown at the location of the groove, which results in a circumstance in which restrictions must be placed on the number of windings of the secondary winding of each winding section, hence a problem that it is difficult to reduce the size of the transformer.
As a high-voltage transformer capable of solving such a problem, the applicant of the present invention proposes a high-voltage transformer (for example, see Japanese Unexamined Patent Publication No. 2004-179587) designed in such a way that in the vicinity of a groove formed in a partition flange, a difference is made between the positions of roll surfaces of neighboring winding sections such that the roll surface of the winding section on the high-voltage side is located outward in radial direction with respect to the roll surface of the winding section on the low-voltage side adjacent to the high-voltage side across the partition flange.
According to this high-voltage transformer, it is possible to place the secondary winding of the top layer of the winding section on the low-voltage side apart from the secondary winding of the top layer of the winding section on the high-voltage side in the vicinity of the groove formed in the partition flange (particularly it is possible to prevent the secondary winding passed from the winding section on the low-voltage side from contacting the secondary winding of the top layer of the winding section on the high-voltage side), and therefore even when the number of windings of the secondary winding of each winding section is increased, dielectric breakdown hardly occurs between the neighboring winding sections. For this reason, it is possible to reduce the total number of winding sections, reduce the size of the high-voltage transformer and at the same time prevent dielectric breakdown from occurring between neighboring winding sections.
In the field of manufacturing electronic devices for which miniaturization processes are being carried forward in short cycles, there is a demand for a further reduction in size of a high-voltage transformer mounted on a circuit board in recent years. Such a demand for downsizing can be roughly divided into two categories; one intended to mainly reduce the mounting area of the high-voltage transformer on a circuit board and the other intended to mainly realize a low-profile high-voltage transformer (reduce the length in direction perpendicular to the surface of the circuit board). Recently, there are an increasing number of cases where the overall volume of the high-voltage transformer is required to be reduced while balancing between a length to width ratio of the mounting area and a low-profile rate.
The high-voltage transformer in Japanese Unexamined Patent Publication No. 2004-179587 above has a tendency that when the direction of the roll is assumed to be lateral direction, the length in this lateral direction is greater than lengths in longitudinal direction (width) or height direction. Therefore, methods for reducing the total number of winding sections or reducing the total length of a roll to reduce the overall size while balancing between length, width and height or the like have been explored. However, to reduce the total number of winding sections while securing a predetermined output voltage, the number of windings of the secondary winding of each winding section cannot help but be increased and this increase in the number of windings in each winding section directly results in an increased diameter of the secondary winding in each winding section. Moreover, in order to provide a sufficient function to prevent dielectric breakdown between neighboring winding sections, it is necessary to increase an amount of displacement in surface positions of respective rolls of the neighboring winding sections by the amount of increase in the diameter of winding of secondary windings of the respective winding sections. Furthermore, when a core (magnetic core) is inserted into the roll, it is also necessary to increase the diameter of the roll itself as the amount of displacement in surface positions of respective rolls to secure the space for the insertion of this core.
The invention disclosed in Japanese Unexamined Patent Publication No. 2004-179587 above assumes that a direction parallel to the surface of the circuit board or a direction perpendicular to the surface of the circuit board is mainly the direction in which a difference is made between the positions of the roll surfaces of the neighboring winding sections. For this reason, the influence of increasing the diameter of the roll itself of each winding section or the winding diameter of the secondary winding to be wound directly results in an increase in the size in the direction in which the difference is made between the positions of roll surfaces. That is, when the displacement direction is parallel to the surface of the circuit board, the mounting area of the high-voltage transformer increases significantly, whereas when the displacement direction is perpendicular to the surface of the circuit board, the high-voltage transformer has a significantly high profile. Therefore, there is a problem that it is difficult to reduce the size of the transformer with balanced length, width and height while preventing dielectric breakdown between neighboring winding sections.